Criticise return of routine: source of eventual happiness (7)
I believe the answer is:
rapture
'happiness' is the definition.
(thesaurus)
'criticise return of routine source of eventual' is the wordplay.
'criticise' becomes 'rap' (I've seen this before).
'return of' says the letters should be written backwards.
'routine' becomes 'rut' (rut is a kind of routine).
'source of' indicates taking the first letters.
The initial letter of 'eventual' is 'e'.
'rut' reversed gives 'tur'.
'rap'+'tur'+'e'='RAPTURE'
(Other definitions for rapture that I've seen before include "Feeling of bliss, elation" , "Intense joy" , "modified for Nanki-Poo?" , "Bliss, intense delight" , "transport" .)